1. Field of the Invention
The invention relates to a novel process for preparing 4,6-bis(difluoromethoxy)pyrimidine derivatives proceeding from 4,6-dihydroxypyrimidine dialkali-metal salts.
2. Background Art
4,6-bis(difluoromethoxy)pyrimidine derivatives are important intermediates for producing herbicides, for example, for producing sulfonylureas (U.S. Pat. No. 4,900,827).
Hitherto, a plurality of processes have been disclosed for preparing 4,6-bis(difluoromethoxy)pyrimidine derivatives.
U.S. Pat. No. 4,900,827 describes a process for preparing 4,6-bis(difluoromethoxy)pyrimidine derivatives in which a 4,6-dihydroxypyrimidine dialkali-metal salt is reacted with chlorodifluoromethane dissolved in acetonitrile or acetone in the presence of 0.05 to 1.1 mol of water per mol of 4,6-dihydroxypyrimidine dialkali-metal salt. A disadvantage of this process is that the reaction is carried out in a heterogeneous, three-phase system. That is to say, acetonitrile or acetone is the liquid phase, the 4,6-dihydroxypyrimidine dialkali-metal salt is a solid and the chlorodifluoromethane is partly dissolved and partly in the gas phase.
European Published Patent Application No. A 0468069 describes a process for preparing 4,6-bis(difluoromethoxy)pyrimidine derivatives proceeding from the corresponding 4,6-dihydroxypyrimidine by reaction with chlorodifluoromethane at normal pressure in the presence of sodium hydroxide dissolved in dioxane. A disadvantage of this process is that an appreciable amount of chlorodifluoromethane is used and, consequently, the process is not ecological. In addition, dioxane is not suitable as a solvent in the production.